Describe and explain changes to the membrane potential of neurones, including: • how the resting potential is maintained • the events that occur during an action potential • how the resting potential is restored during the refractory period
Describe and explain changes to the membrane potential of neurones, including: • how the resting potential is maintained • the events that occur during an action potential • how the resting potential is restored during the refractory period
Answered step-by-step
Changes in the membrane potential of neurons are essential for transmitting electrical signals, enabling communication within the nervous system. Here’s a breakdown of how these changes occur, including the maintenance of the resting potential, the events of an action potential, and restoration of the resting potential during the refractory period.
1. Resting Potential and Its Maintenance
The resting potential is the stable, negative charge inside a neuron when it is not transmitting a signal, typically around -70 mV. This negative charge is maintained through the following mechanisms:
- Sodium-Potassium Pump: The membrane contains sodium-potassium pumps, which actively transport ions across the cell membrane. For each cycle, this pump moves 3 sodium ions (Na⁺) out of the cell and 2 potassium ions (K⁺) in. This creates a net negative charge inside the cell since more positive ions are pumped out than brought in.
- Selective Membrane Permeability: The neuron’s membrane is more permeable to potassium ions than to sodium ions. Potassium leak channels allow K⁺ ions to diffuse out of the cell more easily than Na⁺ ions can enter. This movement of K⁺ out of the cell further contributes to the negative charge inside.
- Large Anionic Molecules: Large, negatively charged molecules, such as proteins and organic phosphates, are trapped inside the neuron and contribute to the negative internal charge.
These combined mechanisms establish and maintain the resting potential, which is essential for the neuron’s readiness to transmit signals.
2. Events During an Action Potential
An action potential is a rapid, temporary change in the membrane potential that allows the neuron to transmit a signal along its length. The action potential occurs in the following phases:
- Depolarization: When a stimulus reaches a certain threshold (usually around -55 mV), voltage-gated sodium channels open, allowing Na⁺ ions to rush into the neuron. This influx of positively charged ions makes the inside of the neuron less negative, causing a reversal of the membrane potential from -70 mV to around +30 mV.
- Repolarization: After reaching the peak of the action potential, the voltage-gated sodium channels close, and voltage-gated potassium channels open. This causes K⁺ ions to flow out of the neuron, making the inside of the cell more negative again and restoring the membrane potential toward the resting level.
- Hyperpolarization: Sometimes, the outflow of K⁺ ions during repolarization causes the membrane potential to become even more negative than the resting potential, resulting in hyperpolarization. This is due to the slow closing of potassium channels, which allows extra K⁺ to leave the cell.
3. Restoration of Resting Potential During the Refractory Period
After an action potential, the neuron enters a refractory period, during which it temporarily cannot fire another action potential. This period has two phases:
- Absolute Refractory Period: During the initial part of the refractory period, the neuron cannot respond to any new stimuli because the sodium channels are inactivated. This ensures that each action potential is a separate, all-or-nothing event and prevents the action potential from traveling backward along the axon.
- Relative Refractory Period: Following the absolute refractory period, there is a relative refractory period, during which the neuron can only respond to a stronger-than-normal stimulus. At this point, most of the sodium channels have returned to their resting state, but the membrane is still hyperpolarized due to the excess K⁺ that has left the cell.
To restore the original ion distribution and fully reestablish the resting potential, the sodium-potassium pump becomes active again, pumping Na⁺ out of the cell and K⁺ back in. This restores the ion gradients needed for the next action potential.
Summary
- The resting potential is maintained by the sodium-potassium pump, selective permeability to K⁺, and large anions within the cell.
- During an action potential, Na⁺ influx causes depolarization, and K⁺ efflux causes repolarization, followed by a brief hyperpolarization.
- The refractory period ensures one-way transmission and allows the sodium-potassium pump to restore the resting potential, preparing the neuron for the next action potential.
These processes ensure that neurons can reliably and quickly transmit signals, which is essential for the rapid communication within the nervous system.
Sourav
